1. Field of the Invention
The present invention is related to drilling and mining processes and, more particularly, but not by way of limitation, to a mining system incorporating hydraulic, borehole mining techniques particularly adapted for the recovery of coal from relatively thin coal seams.
2. History of Related Art
The recovery of coal from coal seams has been the subject of technical development for centuries. Among the more conventional mining techniques, hydraulic mining systems have found certain industry acceptance. Hydraulic mining typically utilizes high pressure water jets to disintegrate material existing in strata or seams generally disposed overhead of the water jets. The dislodged material is permitted to fall to the floor of the mining area and is transported to the mining surface via gravity and/or water in a flume or slurry pipeline. Along these lines, certain developments in Russia included a series of hydro monitors capable of extracting a strip of coal 3 feet wide and 30 to 40 feet in depth within a matter of minutes. The units were designed to be conveyed on a track to the advancing coal face for extracting the coal. The coal would flow downwardly and be transported to the surface via a flume. Similar techniques to this have found commercial acceptance in China, Canada, and Poland, but with only limited attempts in the United States.
Although not as widely accepted in the United States, hydraulic mining methods have been the subject of numerous U.S. patents. U.S. Pat. No. 3,203,736 to Anderson describes a hydraulic method of mining coal employing hydraulic jets of water of unusually small diameter to cut the coal. Such techniques would be particularly applicable to steeply dipping coal seams. Likewise, U.S. Pat. No. 4,536,052 Huffman describes a hydraulic mining method permitting coal removal from a steeply dipping coal seam by utilizing a vertical well drilled at the lowest point of the proposed excavation. Another slant borehole is drilled at the bottom of the coal seam to intersect with the vertical well. High pressure water jets are then used to disintegrate the coal in a methodical fashion with the resulting slurry flowing along the slant borehole into the vertical well. Once in the well, this coal slurry could be pumped to the surface of the mine. While effective in steeply dipping coal seams where gravity would allow the slurry to flow to the vertical well, other techniques would be necessary for more horizontal mining systems. Additionally, U.S. Pat. No. 4,878,712 to Wang teaches the use of water jets to remove horizontal slices of coal within a seam. Through the sequential mining of layers in this manner from top to bottom, the entire seam of coal can be extracted and the mine roof subsides onto the floor without need for artificial roof support.
Another technique for extracting minerals from subterranean deposits is the above referenced borehole mining. Such techniques create minimal disturbance at the mining surface while water jets are used to cut or erode the pay zone and create a slurry down hole. A sump is created below the pay zone to collect the produced cuttings and slurry, which is transported to the surface via a jet or slurry pump. A wide variety of minerals, primarily soft rock formations, may also be mined utilizing this technique. A more recent borehole mining technique is described in U.S. Pat. No. 3,155,177 to Fly wherein a process for under reaming a vertical well and a hydrocarbon reservoir is shown. The technique illustrated therein utilizes electric motors to convert the apparatus from drilling to under reaming.
More conventional techniques are seen in U.S. Pat. Nos. 4,077,671 and 4,077,481 to Bunnelle which describe methods of and apparatus for drilling and slurry mining with the same tool. A related borehole mining technique is shown in U.S. Pat. No. 3,797,590 to Archibald which teaches the concept of completely drilling the vertical well through the portion of the strata to be mined. Separate lines are used for water jet cutting and slurry removal. A progressive cavity pump is used to transport slurry to the surface. In the later improvement (U.S. Pat. No. 4,401,345) the cutting tool is moved independently from the pumping unit. Later developments are shown in U.S. Pat. No. 4,296,970 which describes the use of various types of rock crushers at the inlet of the jet pump. A feed screw on the bottom of the drill string is used to meter the flow of slurry into the orifice of a venturi in association with the rock crusher. In a subsequent development (U.S. Pat. No. 4,718,728), it is suggested to use a tri-cone bit assembly on the end of the tool to reduce the particle size to allow slurry transport. In U.S. Pat. No. 5,197,783 an extensible arm assembly is incorporated to allow the water jet cutting mechanism to extend outwardly from the borehole mining tool to provide more effective cutting in the water filled cavity.
Complementing some mining techniques is the J. H. Fletcher & Co. Model LHD-13 long hole drill unit. This unit consists of a drilling system disposed upon a four wheeled tender car having a drill boom and carriage. Roof jacks are also included and the system is generally used to install in-mine methane drainage boreholes in advance of gassy coal mines.
The above described mining techniques present methods of and apparatus for mineral excavation for sites with specific geological characteristics. In the main such characteristics include steeply dipping coal seams and/or gravity to facilitate transport of the coal to the surface. Transport of the coal, however, is not the only design problem. The distance between the cutting face and the water jet unit increases as material is eroded away. Cutting effectiveness therefore decreases until the unit is moved. These specific design points have been referred to above and are areas of continued technical development. This is particularly true due to the fact that in borehole mining, cutting effectiveness of the water jets also decreases as the cavity becomes larger in size. When the cavity reaches a point that cutting effectiveness diminishes, either another vertical well must be installed to initiate another cavity or the cutting unit needs to be moved closer to the coal face. Also, when a cavity is created in unconsolidated material, subsidence may be created and the cavity may collapse. Borehole mining is, therefore, referred to as a selective mining technique and may not always be suitable for low cost extraction on a large scale basis. Borehole mining is also generally constrained by the ability to remove material from the sump as described above. It would be an advantage therefore to overcome the problems of the prior art by providing a system for horizontal remote mining capable of addressing low cost and effective mineral excavation while effectively utilizing cutting techniques that are consistent with material removal methods therewith.
The present invention provides such an advance over the prior art by utilizing a continuously advancing horizontal remote mining unit that may be disposed within a coal seam close to the face of the coal being eroded. In this manner, a horizontal remote mining unit may develop a horizontal in-seam excavation with improved cutting and slurry removal effectiveness.